Seismic surveys are often used by natural resource exploration companies and other entities to create images of subsurface geologic structure. These images are used to determine the optimum places to drill for oil and gas and to plan and monitor enhanced resource recovery programs among other applications. Seismic surveys may also be used in a variety of contexts outside of oil exploration such as, for example, locating subterranean water and planning road construction.
A seismic survey is normally conducted by placing an array of vibration sensors (accelerometers or velocity sensors called “geophones”) on the ground, typically in a line or in a grid of rectangular or other geometry. Vibrations are created either by explosives or a mechanical device such as a vibrating energy source or a weight drop. Multiple energy sources may be used for some surveys. The vibrations from the energy source propagate through the earth, taking various paths, refracting and reflecting from discontinuities in the subsurface, and are detected by the array of vibration sensors. Signals from the sensors are amplified and digitized, either by separate electronics or internally in the case of “digital” sensors. The survey might also be performed passively by recording natural vibrations in the earth.
The digital data from a multiplicity of sensors is eventually recorded on storage media, for example magnetic tape, or magnetic or optical disks, or other memory device, along with related information pertaining to the survey and the energy source. The energy source and/or the active sensors are relocated and the process continued until a multiplicity of seismic records is obtained to comprise a seismic survey. Data from the survey are processed on computers to create the desired information about subsurface geologic structure.
In general, as more sensors are used, placed closer together, and/or cover a wider area, the quality of the resulting image will improve. It has become common to use thousands of sensors in a seismic survey stretching over an area measured in square kilometers. Hundreds of kilometers of cables may be laid on the ground and used to connect these sensors. Large numbers of workers, motor vehicles, and helicopters are typically used to deploy and retrieve these cables. Exploration companies would generally prefer to conduct surveys with more sensors located closer together. However, additional sensors require even more cables and further raise the cost of the survey. Economic tradeoffs between the cost of the survey and the number of sensors generally demand compromises in the quality of the survey.
In addition to the logistic costs, cables create reliability problems. Besides normal wear-and-tear from handling, they are often damaged by animals, vehicles, lightning strikes, and other problems. Considerable field time is expended troubleshooting cable problems. The extra logistics effort also adds to the environmental impact of the survey, which, among other things, adds to the cost of a survey or eliminates surveys in some environmentally sensitive areas. As a result, wireless acquisition units have been developed to do away with the burdensome nature of cables in such a system. For instance, U.S. Pat. No. 7,773,457, which is hereby incorporated in its entirety by reference as if reproduced herein, describes a system for performing a seismic survey using wireless acquisition units.
When conducting a survey using wireless data acquisition units, it may be advantageous to perform a wireless data read out from such units in real time. As such, survey verification may occur prior to the conclusion of the survey, and the cost and time associated with manual data retrieval of such units may be reduced or eliminated. However, when employing a real time data read out, the bandwidth usage of the wireless communication between modules must be carefully considered so as not to slow the speed at which the survey is conducted.
In turn, a multiplexing regime may be imparted to the wireless modules in the seismic survey so that multiple units within range of one another may simultaneously broadcast and receive data. Examples of such multiplexing regimes include time division, frequency division, code division, etc. For example, U.S. Pat. No. 7,773,457 describes employing multiplexing techniques in a “bucket brigade” type system to perform real time read out of data.